Engines may use a turbocharger to provide boosted intake air for improved engine torque/power output density. The turbocharger may include a compressor coupled to an exhaust-driven turbine. The compressor may experience surge and/or choke depending on operating conditions. Surge may occur during low air mass flow, when the air flow through the compressor stalls or reverse. For example, compressor surge may occur responsive to heavy tip-outs or may occur under high exhaust gas recirculation (EGR) rates. Compressor surge may lead to noise, vibration, and harshness (NVH) issues such as undesirable noise from the engine intake system. Choke may occur when the air flow through the compressor cannot be increased for a given speed of the compressor. For example, compressor choke may occur responsive to aggressive tip-ins from an idle engine speed condition. During choke, the turbocharger cannot provide additional air to the engine, thus the engine power output density is temporarily limited.
Various approaches have been developed to expand the compressor operating range. One example approach including boosting air with a variable geometry compressor (VGC), wherein air flow through the compressor may be adjusted by changing geometry or position of the VGC. As an example, the pattern of air flow into the compressor may be adjusted by adjusting angle of vanes. As another example, air flow through the compressor may be modified with a passive casing treatment including immovable slots and/or ports. During low air mass flow conditions, the slot of the passive casing treatment may provide a path to recirculate partially pressurized air back to the compressor inlet. The recirculated air flowing through the compressor may enable the compressor to operate with a lower air mass flow rate before surge occurs. During high air mass flow conditions, the slots and/or ports of the passive casing treatment may provide a path to short-circuit air flow through the compressor so that the compressor may operate with a higher air mass flow rate before choke occurs. One drawback of passive casing treatment systems is that an effective location for a passive recirculation slot to prevent surge is different from an effective location for a passive recirculation slot to prevent choke.
Another example approach includes the use of an active casing treatment (ACT) for a compressor, such as shown by Sun et al. in U.S. Pat. No. 8,517,664. Therein, a turbocharger includes an active casing treatment, an impeller, a casing, and a diffuser. A controller adjusts a casing sleeve responsive to mass flow conditions relative to a threshold, or based on a pressure differential in the engine system, so that slots in the casing sleeve align with either a surge slot or a choke slot. Air is selectively allowed to flow between the impeller and the compressor inlet responsive to the slot alignment.
In addition to the issues noted above, the inventors herein have also recognized that changes in the geometry or position of the compressor may temporarily disturb engine operating parameters away from their desired setpoints. The transient disturbance may cause NVH and deteriorate engine performance. As an example, engine operating parameters may be controlled by operating actuators via a feedback control loop. Feedback control signals to the actuators may reflect compressor adjustment only when an error in the engine operating parameters has already occurred and has been sensed. In other words, response time of the feedback control loop may be slow. On the other hand, the process of adjusting compressor geometry or position may be fast relative to the response time of the feedback control loop. For example, the process of moving the casing sleeve of an ACT compressor to align with the surge slot or the choke slot may be abrupt and/or discrete. Therefore, the feedback controller may have limited bandwidth to compensate and reduce the disturbance caused by the compressor geometry adjustment, particularly as the controller is tuned to be responsive to numerous other disturbances and may be tuned to provide a certain drive feel responsive to driver-initiated disturbances. As a result, vibration and noise may occur responsive to each compressor geometry adjustment, and also torque disturbances may occur. Engine fuel economy and emission may likewise be affected. In one example, the issues described above may be addressed by a method comprising adjusting an EGR flow via a first actuator and a turbine flow via a second actuator while adjusting a geometry of a compressor, wherein the EGR flow and the turbine flow are adjusted based on the adjustment of the compressor geometry. In this way, disturbance in engine operating parameters responsive to compressor adjustment may be reduced and/or timely compensated.
As another example, engine gas flow and pressure may be controlled by adjusting the EGR flow and turbine flow via a feedback control loop during engine operation. In particular, the EGR flow and the turbine flow may be adjusted respectively by actuating a first actuator and a second actuator with control signals determined based on measured engine gas flow and pressure. Responsive to compressor surge or choke, geometry or position of the compressor may be adjusted via a compressor actuator to expand the compressor operating range. While actuating the compressor actuator to a desired position, the EGR flow and the turbine flow may be concurrently adjusted to offset the disturbance caused by the compressor geometry adjustment. For example, feedforward control signals may be subtracted from the feedback control signals to the first and second actuators while actuating the compressor actuator. The feedforward control signal may be determined based on an expected disturbance in the engine gas flow and pressure. As such, the engine gas flow and the pressure may be instantaneously adjusted responsive to compressor geometry change. The gas flow and the pressure may remain substantially constant (e.g., within 5% of the average) during compressor adjustment, thus reducing engine operating parameters deviation from the desired setpoints. After adjusting the compressor, the engine gas flow and pressure may be controlled by the feedback loop to a desired level. By sending the feedforward control signal to the first and second actuators concurrently with actuating the compressor, disturbances in engine gas flow and pressure may be reduced and can be preemptive of the disturbance onset.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.